1. Field of the Invention
The present invention relates to the field of electronic circuits, and more particularly, to an electronic inductor circuit comprising cascoded transistors, for use in high-speed modem data access arrangements (DAAs).
2. Description of Related Art
FIG. 1 is a block diagram illustrating the major components of a typical analog data modem 2. A modem 2 consists of a Data Access Arrangement ("DAA") 4, a coder/decoder ("CODEC") 14, a Digital Signal Processor ("DSP") 16, and a microcontroller 18. In certain embodiments, the DSP 16 and the microcontroller 18 may be part of the host computer controller.
In small form factor modems, such as PCMCIA card modems, space is a limiting factor for the modem circuit design. Certain circuits from standard half-card modems cannot be directly transferred to smaller modem platforms. One such circuit is the electronic inductor circuit 8 in the DAA 4. The modem DAA circuit 4 provides an interface between the telephone line and the modem chip-set. The DAA 4 includes a voltage isolation transformer or opto-coupler 6 to isolate the modem chip-set from the line, a relay 10 for going on and off-hook, a ring detection and surge protection circuit 12, and an electronic inductor circuit 8. In order to go off-hook and notify a central office that the modem 2 wishes to make a call, the modem must draw loop current from the telephone line. The electronic inductor circuit 8 is the circuit that performs this task. However, while the electronic inductor circuit 8 presents a low DC impedance, it must still maintain a high AC impedance.
The electronic inductor circuit 8 must also sink between 20 mA to 80 mA of loop current in order to keep the telephone line in the off-hook state. This amount of current causes heat to be generated. On a standard half-card modem the heat is easily dissipated due to the use of large components and the free space on the printed circuit board. When moving to a smaller form factor, however, the heat created from the loop current is difficult to adequately dissipate and will cause certain electrical parameters of the circuit to change. Specifically, the AC impedance presented by the electronic inductor 8 will decrease as the modem card heats up. The heat generated will change the impedance of the circuit from its original cold-state value to an operational steady-state value. It takes the circuit approximately five minutes to reach the operational steady-state. The impedance change is illustrated by the graph shown in FIG. 3. After 30 seconds, the over-all electronic inductor circuit impedance (i.e. the impedance seen by the rest of the DAA circuit) is approximately 26 K.OMEGA.. However, after four minutes, the over-all circuit impedance of the electronic inductor circuit decreases to approximately 24.25 K.OMEGA..
In high-speed modem designs, "echo cancellation" is used to allow full duplex (simultaneous send and receive) communications between two modems. Echo cancellation is performed in part by the modem DSP 16 and in part by the DAA 4. Using echo cancellation, a modem detects its own reflected signal (echo) and "cancels" this signal out in order to receive the other modem's signal. As the impedance of the electronic inductor changes, the amount of echo the modem receives changes. The DSP 16, however, is unable to compensate quickly enough to prevent transmission errors from seriously degrading modem performance.
A typical prior art electronic inductor circuit 30 is shown in FIG. 2. Two transistors 441, 442 are arranged in a common Darlington pair configuration 44. Two resistors 34, 36 bias the Darlington pair such that it is active (i.e. turned on). A DC loop current from the phone line (attached at an input node 32) is drawn through the transistor 44 and an emitter resistance 40 to ground 42. A capacitor 36 shorts the base of the Darlington pair transistor 44 so that any AC signal is grounded. Typical component values for the electronic inductor circuit are:
R.sub.34 =62K.OMEGA. R.sub.38 =75K.OMEGA. R.sub.40 =51K.OMEGA. PA1 C.sub.36 =10 .mu.F Q.sub.44 =CXTA27 (Central Semi. part number)
The impedance of a conventional prior-art electronic inductor circuit 30 looking into the collector of the Darlington pair 44 has been thought to be several orders of magnitude greater than any other resistive elements in the circuit. If, in fact, the impedance of the electronic inductor circuit were high, slight changes in the impedance caused by heat would have little affect on the hybrid tuning. However, measurements on the electronic inductor circuit used in commercial half-card modem designs show that the impedance looking into the collector of the Darlington pair typically ranges from between 0K and 60K ohms. At these relatively low impedance levels, a slight change in magnitude can adversely affect the hybrid tuning, in turn causing a significant degradation in modem performance, especially for high-speed modems. The modem DSP 16 is unable to compensate rapidly enough for the changes caused by the impedance change of the electronic inductor 8, causing a reduction in modem performance. Thus, the impedance of the electronic inductor is a critical parameter in the hybrid tuning of a modem DAA, and therefore any heat-induced impedance changes need to be minimized.
One attempted prior solution replaced the Darlington bipolar transistor with a MOSFET device. This circuit design exhibited similar problems, however. Another solution uses either an op-amp or a custom ASIC to increase or decrease the amount of base current in the Darlington transistor in order to adjust the impedance of the electronic inductor circuit. This solution is too expensive, however, since it requires adding significant extra circuitry.